The invention relates to a method and arrangement for detecting a watermark embedded in an information signal. The invention also relates to a method of removing a watermark from an information signal having an embedded watermark.
Watermarks are perceptually invisible messages embedded in information signals such as multimedia material, e.g. audio, still pictures, animations or video. Watermarks can be used to identify the copyright ownership of information. They allow a copyright owner to trace illegal copies of his material by inspecting whether his watermark is present in said copies.
Watermarks are embedded in an information signal by modifying data samples of the signal (e.g. audio samples of an audio signal, pixels of an image, transform coefficients of a transform-coded signal, etc.) such that the original is not perceptibly affected. Various methods of watermarking are known in the art. For example, pixels of an original image are slightly incremented or decremented in accordance with corresponding bits of a binary watermark pattern.
In order to detect whether an information signal has an embedded watermark, the signal is subjected to a statistical analysis. The statistical analysis yields a parameter, hereinafter referred to as xe2x80x9cdecision variablexe2x80x9d, which indicates to which extent the watermark is present in the signal. For example, if an image signal is watermarked by incrementing or decrementing its pixels in accordance with a watermark pattern, the decision variable may be the amount of correlation between the signal and an applied reference copy of the watermark. If an image is watermarked by modifying selected pixels, a prediction for said pixels is calculated from temporally or spatially adjacent pixels. The decision variable may then be the number of pixels being sufficiently different from their prediction.
Prior art watermark detectors generate a binary output signal indicating xe2x80x9cwatermark foundxe2x80x9d or xe2x80x9cno watermark foundxe2x80x9d. This is achieved by comparing the decision variable with a predetermined threshold. If the value of the decision variable exceeds the threshold, the watermark is considered to be present in the signal. In consumer products such as home recorders, the watermark detector will generally be implemented as a tamperproof box, so that an attacker can neither reversely engineer the detection algorithm nor its implementation parameters. It has been found, however, that an attacker can nevertheless remove a watermark by observing the detector""s binary output signal under various input signal conditions.
It is an object of the invention to provide a method and arrangement for detecting a watermark which is less vulnerable to attacks.
To this end, the method in accordance with the invention is characterized by the step of randomly generating said output signal for decision values below said threshold. Preferably, the random output signal is generated for a range of decision values between said threshold and a further predetermined threshold.
The invention is based on the recognition that the prior art watermark detectors exhibit a sharp transition between the decisions xe2x80x9cwatermark foundxe2x80x9d and xe2x80x9cno watermark foundxe2x80x9d. This property allows an attacker to iteratively modify an input signal and observe the detector""s output until he has found an input signal which causes the detector to operate in the vicinity of its threshold. Having thus found the transition point, it is not difficult to generate an input signal which closely resembles the watermarked signal but is not recognized as being watermarked. By randomizing the transition point of the detector, the attacker acquires less (or at least less reliable) information from each signal modification.
Further advantageous embodiments of the invention are defined in the dependent claims.